THE NUCLEI OF UROLEPTUS MOBILIS 309 



number, a considerable percentage of individuals have from ten 

 to twelve or even fourteen macronuclei during the early stages. 

 I have not seen one with less than eight. 



The first change undergone by these nuclei, as in preparation 

 for division, is the loss of the nuclear cleft, and it is in this .phase 

 that the increase in number occurs, if at all. The finer structure 

 at this time is finely granular and homogeneous (figs. 24 to 27). 



As conjugation progresses a few scattered, larger granules 

 appear in each nucleus. These increase in number and in size 

 until, at the period of nuclear interchange, the granules are from 

 1 M to 2 M in diameter (figs. 68 and 69). The membranes are 

 apparently lost, and what were homogeneous finely granular nu- 

 clei are now mere masses of large irregular chromatin bodies. 

 The fixing fluids evidently have something to do with the ap- 

 parent disappearance of the membranes, for they are apparent 

 in all stages of conjugants fixed with Flemming, while the gran- 

 ules are always smaller than in material fixed with sublimate 

 acetic. The nuclei are in this condition of granular disintegra- 

 tion at the time of separation of the conjugating individuals, and 

 remain so for an hour or more after separation (figs. 76 and 80). 

 This stage, however, is soon replaced by a highly characteristic 

 phase. The masses of large granules again fuse, forming eight 

 homogeneous, non-granular, and smooth spherical nuclei with an 

 intense staining capacity. These slowly fade away in the cyto- 

 plasm, first one, then another disappearing, so that ex-conjugants 

 with 8, 7, 6, 5, 4, 3, 2, and 1 degenerating macronuclei are found 

 (figs. 86 to 90). Occasionally a mass of fine granules marks the 

 spot where a nucleus has disappeared, but usually it vanishes 

 without leaving a trace. The first evidence of degeneration is 

 loss of staining capacity; six or seven of the nuclei may stain in- 

 tensely while the others appear pale and ghost-like. Or four or 

 five will stain deeply and there will be no trace of the others. 

 Finally, at 96 hours after separation the old macronuclei are 

 entirely absorbed in the cytoplasm and the new macronucleus 

 begins to divide and to form the eight characteristic nuclei of 

 the vegetative phases. 



